1. Field of the Invention
The present invention generally relates to a lubricant and more specifically to a lubricant having advanced thermal conductivity due to the nano-graphite plates included therein.
2. The Prior Arts
In general, the monolayer graphite, also called graphene, has a lattice structure formed by a monolayer of carbon atoms, which are tightly packed in two-dimensional honeycomb crystal lattice by the graphite bond (sp2). Thus, the monolayer graphite has a thickness of one carbon atom. The graphite bond is a composite chemical bond derived from the covalent bond and the metallic bond, such that graphene is a perfect substance possessing both key properties of an insulator and a conductor. In 2004, Andre Geim and Konstantin Novoselov at the University of Manchester in the UK successfully proved that graphene is obtained from a piece of graphite by using adhesive tape, and were thus awarded the Nobel Prize in Physics for 2010. Graphene is the thinnest and hardest material in the world now. It has thermal conductivity greater than that of carbon nanotube and diamond. Its electron mobility at room temperature is higher than the carbon nanotube and silicon crystal. Also, the electric resistivity of graphene is even lower than that of copper or silver, and so far is considered as the material with the lowest resistivity.
In the prior arts, graphene can be produced by three methods, including graphite exfoliating, direct growth and carbon nanotube transformation. Especially, the graphite exfoliating method is used to form graphene powder. The most suitable method for mass production is the method of redox reaction. Specifically, the graphite material is first oxidized to form graphite oxide, and it is then processed by separation and reduction reaction to obtain graphene.
US Patent Publication No. 20050271574 disclosed a process for producing graphene, which includes the steps of first performing intercalation by strong acid on a piece of natural graphite, primarily exfoliating the piece of natural graphite by suddenly contacting with a high heat source, and then completely exfoliating the piece of natural graphite by using high energy grinding balls so as to form graphene powder. Whatever method is used to produce graphene powder, owing to the nanometer structure of graphene, the present process is not only complicated and is badly polluted, but the tap density of the manometer material is also much lower. For example, the tap density is much less than 0.01 g/cm3, and the resultant volume is much larger such that it is possible to aggregate by Van der Waals' forces. Therefore, it is a challenge for mass production or industrial application even graphene possesses such excellent physical properties, and it is easy to cause negative effect on derivative products.
U.S. Pat. No. 8,222,190 disclosed a modified lubricant consisting of nano graphene. The embodiment disclosed that the friction coefficient of the lubricant having 5 wt % graphene is less than half to the friction coefficient of the lubricant having 5 wt % nano graphite powders or nano carbon tubes. However, the performance appears under the condition of the amount of the graphene greater than 5 wt % due to the aggregate of graphene.